Compact heat exchange systems using, for example, a burner positioned inside a plenum formed by a heat exchanger which has a surface area contacting the products of combustion which is substantially larger than the surface area contacting a fluid to be heated by the products of combustion can economically extract heat from the products of combustion. Highly efficient systems have developed to capitalize on compact heat exchange systems and cope with the concurrent problem of condensation deposits. Examples of such systems are described in U.S. Pat. Nos. 3,967,590, issued July 6, 1976; 3,997,109, issued Dec. 14, 1976; and 4,135,487, issued Jan. 23, 1979. They are shown combined with cooling systems in package units for external mounting, for example, in the backyard or on the roof of a home.
Being installed outside the home or other enclosure, such systems are subjected to greatly varied operating conditions. Rather than combusting fuel with air of a fairly constant temperature and humidity, they must combust whatever the outside environment offers. Hence, they are subjected to mixtures of greatly different enthalpies. This is further aggravated by the need to combust different fuels stored at varied temperatures depending upon the location and type of installation.